The present invention relates to a cabin communication system without acoustic echo cancellation that nevertheless provides echo rejection to enable a voice spoken within the cabin to be increased in volume for improved understanding without creating any unwanted noise amplification. The present invention also relates to a movable cabin that advantageously includes such a cabin communication system for this purpose. In this regard, the term xe2x80x9cmovable cabinxe2x80x9d is intended to be embodied by a car, truck or any other wheeled vehicle, an airplane or helicopter, a boat, a railroad car, turboprop aircraft and indeed any other enclosed space that is movable and wherein a spoken voice may need to be amplified or clarified.
As anyone who has ridden in a mini-van, sedan or sport utility vehicle will know, communication among the passengers in the cabin of such a vehicle is difficult. For example, in such a vehicle, it is frequently difficult for words spoken by, for example, a passenger in a back seat to be heard and understood by the driver, or vice versa, due to the large amount of ambient noise caused by the motor, the wind, other vehicles, stationary structures passed by etc., some of which noise is caused by the movement of the cabin and some of which occurs even when the cabin is stationary, and due to the cabin acoustics which may undesirably amplify or damp out different sounds. Even in relatively quiet vehicles, communication between passengers is a problem due to the distance between passengers and the intentional use of sound-absorbing materials to quiet the cabin interior. This communication problem is frequently compounded by the simultaneous use of high-fidelity stereo systems for entertainment, whether those systems are traditional audio systems or, as is coming into prevalence now, television and/or computer systems that provide an audio output.
To amplify the spoken voice, it may be picked up by a microphone and played back by a loudspeaker. However, if the spoken voice is simply picked up and played back, there will be a positive feedback loop that results from the output of the loudspeaker being picked up again by the microphone and added to the spoken voice to be once again output at the loudspeaker.
To avoid an echo due to the reproduced voice itself, prior art approaches have generally used acoustic echo cancellation, such as for example in U.S. Pat. No. 5,602,928, which includes active noise control (ANC), or in U.S. Pat. No. 5,706,344, which does not include ANC. In this type of system, an acoustic echo cancellation apparatus can be coupled between the microphone and the loudspeaker to remove the portion of the picked-up signal corresponding to the voice component output by the loudspeaker. This is possible because the audio signal at the microphone corresponding to the original spoken voice is theoretically highly correlated to the audio signal at the microphone corresponding to the reproduced voice component in the output of the loudspeaker.
Any reproduced noise components may not be so highly correlated and need to be removed by other means, for example by means of an appropriate noise filter.
These systems attempt to selectively receive speech from one location (e.g. by a microphone from where a first person in the vehicle is talking), enhance that speech by removing noise and entertainment stereo interference, amplify the enhanced speech and reproduce it in another location (i.e. from a loudspeaker directed at a second person in the vehicle). It is highly desirable not to degrade either the quality of the speech or the quality of the entertainment stereo output.
In an enclosed cabin with such prior art systems, the microphone and the loudspeaker are not acoustically isolated, i.e. the microphone is in a position to pick up the output of the loudspeaker. As noted above, the acoustic echo cancellation system is intended to estimate and remove from the microphone signal that part which is due to speech, music, noise etc. that was just played out of the loudspeakers. It is particularly important that the music (or other sounds) from the high-fidelity stereo system be cancelled. Any uncancelled music will be reproduced in the cabin from a loudspeaker at a location spaced from the original stereo system loudspeaker. This results in the detriment of stereo separation, fade and balance. It may introduce unpleasant reverberation due to processing delays, where the delay in picking up the music by the microphone, processing it and then re-outputting it by the loudspeakers may be long enough to be detectible by the human ear. Therefore, the connection of the high fidelity stereo system to the acoustic echo cancellation system contributes to the cabin communication system cost.
FIG. 1 is a simplified block diagram of a conventional cabin communication system (CCS) 100 using a microphone 102 and a loudspeaker 104. As shown in the figure, a conventional echo canceller 106 and a conventional speech enhancement filter (SEF)108 are connected between the microphone 102 and loudspeaker 104. A summer 110 subtracts the output of the echo canceller 106 from the input of the microphone 102, and the result is input to the SEF 108 and used as a control signal therefor. The output of the SEF 108, which is the output of the loudspeaker 104, is the input to the echo canceller 106. In the echo canceller 106, on-line identification of the transfer function of the acoustic path (including the loudspeaker 104 and the microphone 102) is performed, and the signal contribution from the acoustic path is subtracted at summer 110.
As disclosed in a commonly-assigned U.S. patent application, a further development of such a system, illustrated in FIG. 2, includes a novel and unobvious echo canceller 112 and a novel and unobvious speech extraction filter in the form of a special Wiener filter 114, and further includes a specific input from the car""s (vehicle""s) audio system. This input is fed to a stereo gain estimator 116, such as a conventional single tap LMS, which estimates a cancellation signal to be fed to the echo canceller 112 through summer 118 under the control of the output of summer 110.
However, there are certain problems associated with the use of an acoustic echo cancellation (AEC) system in a cabin communication system. First, the AEC must be adaptive to accommodate changes in the acoustic environment, and second it provides cancellation proportional to the length of its filters. An AEC will always have some residual, i.e. partially uncancelled, signal that may be perceptually unpleasant or contribute to unintelligibility. Long AEC filter length and adaptivity impose a significant computational cost.
The requirement of adaptation on the AEC also poses a problem in system stability. The AEC must adapt to changes in the environment, e.g. passenger movement, such that it always provides enough cancellation. Insufficient cancellation results in a loop gain greater than one and unpleasant acoustic screeching.
A further problem with known cabin communication systems (CCS) is the need for microphone independence. The microphones of known CCS""s are often responsive to not only the desired location, but also to other locations as well. This has the undesirable effect that a person hears an amplified version of his own speech. Ideally, each person should hear only the amplified versions of the other passengers"" speech.
One approach to solve this problem is found in the above-mentioned U.S. patent application using acoustic echo cancellation, wherein a plurality of microphones are used at each location and each plurality is beamformed to improve the signal to noise ratio of the person speaking, to reject some of the stereo signal from the stereo system loudspeakers and to provide some microphone independence. There are many well known beamforming techniques and adaptive microphone arrays, although some do not relate to spatial filtering and no conventional system is known to be applied to the particular requirements of a CCS.
Accordingly, it is an object of the invention to provide a cabin communication system that avoids the problems of the prior art.
It is another object of the invention to provide a cabin communication system that provides echo cancellation without using an acoustic echo cancellation system.
It is yet another object of the invention to provide an adaptive nulling microphone array that is advantageously used in a cabin communication system of a moving vehicle.
It is a further object of the invention to provide a moving vehicle including a cabin communication system incorporating an advantageous adaptive nulling microphone array in the moving vehicle.
It is yet a further object of the present invention to provide a cabin communication system which effectively rejectes sounds from, and therefor does not interfere with, audio entertainment systems.
In accordance with these and other objects of the invention, a cabin communication system for improving clarity of a voice spoken within an interior cabin includes a microphone array, the microphone array including a first microphone, positioned at a first location within the cabin, for receiving the spoken voice primarily in a first direction and for converting the spoken voice into a first audio signal, and a second microphone, positioned at a second location within the cabin, for receiving the spoken voice primarily in the first direction and for converting the spoken voice into a second audio signal. A sound source is provided that inputs sound into the cabin such that the input sound approaches the microphone array primarily in a second direction different from the first direction, and a processor combines the first and second audio signals to provide a resultant audio signal. The combining of the first and second audio signals defines a beampattern of the microphone array that includes a plurality of lobes and a plurality of nulls such that the spoken voice is primarily received by the microphone array along the first direction at a first one of the plurality of lobes and such that the input sound is primarily received by the microphone array along the se direction at a first one of the plurality of nulls, whereby any component in the resultant audio signal indicative of the input sound is substantially minimal. A loudspeaker then converts the resultant audio signal into an output reproduced voice within the cabin.
Advantageously, the first and second microphones define a beamformed phase array and the first one of the plurality of lobes is a main lobe of the beampattern. The sound source can be the loudspeaker, and the cabin communication system may further comprise a second sound source, such as the speaker of an entertainment system, that Inputs a second sound directionally into the cabin such that the input second sound approaches the microphone array primarily in a third direction different from both the first and second directions, wherein the input second sound is primarily received by the microphone array along the third direction at a second one of the plurality of nulls, whereby any component in the resultant audio signal indicative of the input second sound is substantially minimal.
In accordance with another aspect of the invention, there is provided a movable cabin including means for moving the cabin and a cabin communication system as indicated above.